1. Field of the Invention
This invention relates to a metal gasket used for sealing opposed fitting surfaces between a cylinder head and a cylinder block in a multiple-cylinder engine, and its production method.
2. Description of the Prior Art
A metal gasket made of a metal material has been used conventionally to seal opposed fitting surfaces between a cylinder head and a cylinder block of an engine. The metal gasket generally has beads in the proximity of holes for combustion chamber, water holes and oil holes of the structural members described above, and when it is fixed by fastening the cylinder head and the cylinder blocks by bolts, etc., the beads form elastic annular contact portions with respect to the opposed fitting surfaces, and seal them around the above holes.
However, a higher output and a smaller weight have been required for recent engines and as one of means for satisfying the requirements, the cylinder head, the cylinder block, etc., tend to be produced by using an aluminum material having a small specific gravity in place of the conventional materials having a great specific gravity such as a steel and a casting. Though light in weight, the aluminum material has low rigidity. Therefore, troubles are likely to occur at the time of fastening of the metal gasket and during the operation of the engine. In other words, the positions of the fastening bolts of the metal gasket are dispersed to outer peripheral portions or relatively to the outer peripheral portions of the metal gasket and are not always disposed uniformly around the holes. Therefore, when the opposed fitting surfaces of the structural materials having the low rigidity are fastened by the bolts through the metal gasket consisting of a single plate, the opposed fitting surfaces are likely to become non-uniform. As a result, a high temperature high pressure combustion gas enters between the opposed fitting surfaces of portions having large distortion, such as the portions between the holes, corrodes and contaminates the beads of the metal gasket interposed between the opposed fitting surfaces, and lowers the seal effect.
Furthermore, since rigidity of these structural materials lowers, relative displacement of the cylinder head to the cylinder block becomes great in accordance with combustion cycles during the engine operation, the gap between the cylinder head and the cylinder block repeatedly increases and decreases, and the temperature of the metal gasket changes greatly in accordance with the operation and stop of the engine. Accordingly, repeated stresses, that is, mechanical stress and thermal stress, act on the metal gasket, too. This load fluctuation stress is most pronounced at portions having the lowest rigidity of the cylinder block and the cylinder head, so that the beads undergo buckling or cracks develop, thereby deteriorating seal performance.
The Applicant of the present invention previously filed a patent application relating to a single plate metal gasket disposed and fastened between the opposed fitting surfaces of the cylinder head and the cylinder block of the engine, and its production method (Japanese Patent Application No. 306295/1990). This metal gasket includes holes formed in a elastic metal plate, beads spaced apart outward in a radial direction from the periphery of the holes and extending along the holes, and folded portions formed by folding the metal plate on the convexed surface side of the beads around the holes and outward in the radial direction. The folded portions of the metal plate are shaped into a predetermined thickness, and are heat-treated. Therefore, when the elastic metal plate is fastened and brought into a pressed state between the opposed fitting surfaces, the beads form annular seal portions with respect to the opposed fitting surfaces and at the same time, the folded portions which have about twice the thickness of the metal plate form another seal portions around the holes, and the beads and the folded portions offset any irregularity of the opposed fitting surfaces, suppress a distortion quantity of the cylinder head occurring due to the repetition of the combustion cycles, reduces the fluctuation load stress, and prevent complete compression of the beads.
In the metal gasket described above, the reference discloses a structure wherein a metal plate having a thickness smaller than that of the metal plate described above is laminated on the external side of the beads other than the portions between the holes in a radial direction and on the convexity surface side of the beads, and an offset between the folded portions and regions which are more outwards than the beads in the radial direction is regulated, a structure wherein a soft metal plate is clamped in the folded portions and folded back when the offset is below a necessary offset, a structure wherein the folded portions are so formed as to be thick in the regions between the holes and to be thin in other regions so as to obtain a high push force in the regions between the holes at which high seal performance is required, and a structure wherein a non-metallic material such as heat- and oil-resistant rubber or resin is applied to both surfaces of the elastic metal plate in order to avoid the direct metal-to-metal contact with the opposed fitting surfaces, to prevent corrosion and contamination of the metal surface and to secure the seal function irrespective of concavo-convexities of the opposed fitting surfaces.
The production method of the metal gasket described above comprises a step of forming in the elastic metal plate perforations having an inner diameter smaller than that of the holes that would have been finished; a step of forming beads along the periphery of the perforations in such a manner as to be spaced outward in the radial direction from the periphery of the perforations; a step of forming folded portions having folded base portions serving as cylinder bore holes by bending outward the metal sheet in the radial direction on the convexity side of the beads around the perforations; a step of applying a compressive force to the folded portions and shaping them into a predetermined thickness; and heat-treating the metal plate having the folded portions formed thereon.
However, the metal gasket described above forms basically the perforations having an inner diameter smaller than that of the holes and includes the elastic metal plate having the folded portions formed by folding outward the plate in the radial direction around the perforations. Accordingly, such a elastic metal plate has a structure where the folded portions are completely folded 180.degree. without a gap at the folded base end portions serving as the holes, and receive a servere stress change at the time of machining. Therefore, damage such as cracks and breakage is likely to occur at the folded portions, or the metal plate is exposed to the repeated stress in accordance with the combustion cycles for a long time after being fixed between the opposed fitting surfaces, even though it is heat-treated, and cracks are likely to occur at the folded base portions.
When the folded portions are formed around the holes, an offset portion corresponding to the thickness of the plate is formed between the periphery of the hole and in a region which is outward in the radial direction from the periphery. If this offset portion is great, the elastic metal plate of the gasket main body compulsively undergoes deformation in the regions inside and outside this offset portion when the metal gasket is fastened between the opposed fitting surfaces of the cylinder head and the cylinder block. This deformation region is originally disadvantageous stress-wise, and cracks, etc., are likely to occur because more compulsive stress acts on it due to the load fluctuation in accordance with the combustion cycles of the engine. Furthermore, when the elastic metal plate has the beads formed into a profile shape extending along the holes for combustion chamber and located outward from the folded portions in the radial direction, strong seal portions are defined at the time of fastening and prevent the leak of the corrosive, high temperature combustion gas from the holes for combustion chamber if the thickness of the folded portions is great, and the role of sealing of the portions having the beads formed thereon is eased that much. Accordingly, the burden of sealing concentrates excessively on the folded portions, and when the folded portions are broken during the use for a long time, the sealing of the beads which is not much strong cannot exhibit effectively the functions of preventing the leak of the combustion gas
When the offset portion of the elastic metal plate by the folded portions described above is higher than a desired height, the height of the offset portion of the folded portions can be regulated to decrease it by laminating a thickness regulation plate on the elastic metal plate, but according to such a regulation method, the regulation plate must be prepared separately. Therefore, the production cost of the metal gasket rises, and when the regulation plate is produced, it must be provided with the shape which coincides with the machining shapes of the beads, etc., of the elastic metal plate. In addition, a lamination on the elastic metal plate becomes necessary, and an apparatus for the lamination becomes also necessary. For these reasons, this method is troublesome and is not economically advantageous.
The free ends of the folded portions are not particularly subjected to chamfering, etc., and come as such into contact with the, surface of the elastic metal plate. Since the elastic metal plate is exposed to the environment in which the stress acts repeatedly for a long time, cracks and traces of friction occur on the surface of the flexible metal plate.
In the production method of the metal gasket, the metal gasket involves the limitations of the thickness, hardness and elongation characteristics of the elastic metal plate used. As to the thickness of the elastic metal plate, the thickness of the folded portions is not adjusted in advance. Therefore, fold-machining must be carried out using the thickness of the elastic metal plate used, and the force required for machining is great, so that a drastic change of the plate thickness occurs at the folded base portions and the folded portions are likely to crack. In the case of so-called "pre-compression" where the plate thickness is adjusted by a press, etc., after the elastic metal plate is folded, the adjustment quantity becomes great. Furthermore, since it is difficult to improve precision of the thickness adjustment and flatness, there is a limit to the thickness of the plate that can be employed.
In connection with the hardness of the elastic metal plate, a material having a high hardness cannot be folded easily. Therefore, a elastic metal plate which is relatively soft with a hardness of below Hv 200 before fold-machining and which can be machined easily but keeps its hardness must be selected and used. In this point, too, there is a limitation to the hardness of the elastic metal plate that can be employed. As to the elongation characteristics of the elastic metal plate, cracks, breakage, distortion, etc., are likely to occur at the folded portions during fold-machining if a material having inferior elongation characteristics is used. Accordingly, an expensive flexible metal plate having good elongation characteristics must be employed in order to prevent the occurrence of cracks, breakage, distortion, and so forth.